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  • Atrioventricular (AV) block occurs due to delayed conduction of an impulse between the atria or ventricles, due to intrinsic or extrinsic causes.
    • Causes of Atrioventricular (AV) Block

    • Intrinsic

    • Idiopathic degeneration
    • Myocardial infarction
    • Congenital - congenital heart disease, neonatal lupus
    • Infiltrative disease - sarcoidosis, amyloidosis, haemochromatosis
    • Cardiac procedures - valvular surgery, correction of congenital heart disease, catheter ablation
    • Systemic lupus erythematosus
    • Myocarditis
    • Extrinsic

    • Physiologic - sleep, athletes
    • Autonomic - carotid sinus massage, carotid sinus hypersensitivity
    • Drugs - beta blockers, calcium channel blockers, digoxin, adenosine, amiodarone
    • Hyperkalaemia
    • Hypothyroidism
  • Physiologic and autonomic causes of AV block tend to result in first degree or Mobitz I heart block, while pathologic causes tend to result in Mobitz II, advanced second degree or complete heart block.

1st Degree AV Block

  • First degree AV block indicates delayed conduction between the atria and the ventricles.
    • Look For

    • Prolonged PR interval >200ms (5mm).
    • 1st Degree AV Block

2nd Degree AV Block: Type I Mobitz

  • Also known as Wenckebach AV block.
  • Type I second degree AV block occurs when there is progressively delayed AV transmission eventually resulting in a missed QRS complex.
    • Look For

    • Initial normal PR interval with progressive prolongation of PR interval followed by a dropped QRS complex.
    • 2nd Degree AV Block: Type I Mobitz

2nd Degree AV Block: Type II Mobitz

  • Type II second degree AV block occurs when there is an intermittently missed QRS complex due to failure of conduction through the AV node.
    • Look For

    • Constant PR interval with intermittently dropped QRS complexes.
    • 2nd Degree AV Block: Type II Mobitz

Advanced 2nd Degree Heart Block

  • In advanced or ‘high-grade' second degree heart block, multiple beats are blocked and it is difficult to distinguish between Mobitz type I and II.
    • Look For

    • Two or more consecutive P waves without QRS complexes
    • May be in a 3:1, 4:1 or higher pattern
    • Advanced 2nd Degree Heart Block

3rd Degree AV Block

  • Complete atrioventricular block represents complete failure of conduction between the atria and ventricles.
    • Look For

    • Complete dissociation of P waves and QRS complexes.
    • 3rd Degree AV Block
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 Mangrum JM, DiMarco JP. The evaluation and management of bradycardia. New England Journal of Medicine. 2000 Mar 9;342(10):703-9.